The human spine includes intervertebral discs that are located between adjacent vertebrae of the spine. The intervertebral discs function to stabilize the spine and distribute forces between vertebrae. Intervertebral discs generally comprise three regions, known as the annulus fibrosis, the nucleus pulposus, and the cartilagenous, or bony, end plates.
The nucleus pulposus retains a gelatinous consistency, and includes a high proteoglycan content. The nucleus pulposus further retains approximately 70% to 90% water, aiding in its fluid nature. The nucleus pulposus is contained within the annulus fibrosis. The annulus fibrosis retains a more rigid consistency, and is composed primarily of type I and type II collagen. The annulus fibrosis functions to provide peripheral mechanical support to the intervertebral discs, torsional resistance, and resistance to the hydrostatic pressures of the nucleus pulposus.
Intervertebral discs may be displaced or damaged due to trauma or disease. Disruption of the annulus fibrosis may allow the nucleus pulposus to protrude into the vertebral canal, a condition commonly referred to as a herniated or ruptured disc. The extruded nucleus pulposus may press on a spinal nerve, resulting in nerve damage, pain, numbness, muscle weakness, and paralysis. Intervertebral discs also may deteriorate due to the normal aging process. As a disc dehydrates and hardens, the disc space height may be reduced, leading to instability of the spine, decreased mobility, and pain.
One way to relieve the symptoms of these conditions is by surgical removal of a portion or all of the intervertebral disc. The removal of the damaged or unhealthy disc may allow the disc space to collapse, which could lead to instability of the spine, abnormal joint mechanics, nerve damage, and as severe pain.
Researchers therefore have investigated the efficacy of implanting an intervertebral disc prosthesis to replace the damaged portion of the patient's intervertebral disc. One such prosthesis is an artificial implantable nucleus replacement device. Nucleus implants are used when the nucleus pulposus of the intervertebral disc is damaged but the annulus fibrosis and vertebral end-plates are still sufficiently healthy. Nucleus replacement surgery involves removing the damaged nucleus pulposus of the intervertebral disc and insertion of the nucleus implant inside of the retained annulus fibrosis. The nucleus implant can be a molded polymer-containing device designed to absorb the compressive forces placed on the spine. For increased strength, the nucleus implant may be combined with an internal matrix of, for example, bio-compatible fibers. The retained annulus fibrosis provides tensile strength. Some desirable attributes of a hypothetical intervertebral disc prosthesis include axially compressibility for shock absorbance, excellent durability to avoid future replacement, and biocompatibility.
The description herein of problems and disadvantages of known apparatus, methods, and devices is not intended to limit the invention to the exclusion of these known entities. Indeed, embodiments of the invention may include one or more of the known apparatus, methods, and devices without suffering from the disadvantages and problems noted herein.